Transmitter having voltage driver and current driver

ABSTRACT

A circuit includes a first power node at a first voltage level, a second power node at a second voltage level, a first voltage driver, a first current driver, and a control unit. The first voltage driver is configured to electrically couple a first output node to the first power node when a first input signal at the first input node is at a first logic state, and electrically couple a first output node to the second power node when the first input signal is at a second logic state. The first current driver is configured to inject or extract a first adjustment current into or out of a first output node. The control unit is configured to generate a measurement result of the first voltage level, and to set the first adjustment current according to the measurement result.

BACKGROUND

A signal transmission system includes a transmitter, a receiver, and a channel connecting the transmitter and the receiver. For transmitting a signal along the channel having a predetermined intrinsic impedance, a voltage mode driver is usable as the transmitter and is capable of outputting a voltage signal having a voltage level changing between (a) one-half of a difference between a first supply voltage (having a voltage level VDD) and a second supply voltage (having a voltage level GND) and (b) the voltage level GND of the second supply voltage. Thus, a signal swing of the output signal is (VDD−GND)/2. In some applications, two voltage mode drivers are used side by side to transmit a pair of differential signals, and the overall signal swing of the output different signals would be (VDD−GND). In some applications, the signal transmission system is designed to be in compliance with an industrial standard, where a defined range of output signal swing is also provided.

DESCRIPTION OF THE DRAWINGS

One or more embodiments are illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout.

FIG. 1 is a circuit diagram of a signal transmission system in accordance with one or more embodiments.

FIG. 2A is a circuit diagram of a voltage driver and a current driver having an input signal at a logic high state in accordance with one or more embodiments.

FIG. 2B is a circuit diagram of a voltage driver and a current driver having an input signal at a logic low state in accordance with one or more embodiments.

FIG. 3A is a circuit diagram of a set of current sources usable in a current driver in accordance with one or more embodiments.

FIG. 3B is a circuit diagram of another set of current sources usable in a current driver in accordance with one or more embodiments.

FIG. 4 is a flow chart of a method of operating a transmitter in accordance with one or more embodiments.

DETAILED DESCRIPTION

It is understood that the following disclosure provides one or more different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, examples and are not intended to be limiting. In accordance with the standard practice in the industry, various features in the drawings are not drawn to scale and are used for illustration purposes only.

Moreover, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top,” “bottom,” “left,” “right,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features.

In some embodiments, output signal swing is determined based on a voltage level of a power supply voltage of a voltage driver and an adjustment current injected into or extracted from an output node of the voltage driver. The output signal swing is adjustable to have a predetermined signal swing by controlling an amount and direction of the adjustment current based on a measurement result of the voltage level of the power supply voltage.

FIG. 1 is a circuit diagram of a signal transmission system 100 in accordance with one or more embodiments. Signal transmission system 100 includes a transmitting portion (i.e., a transmitter) TX, a receiving portion (i.e., a receiver) RX, and a channel portion CH between transmitting portion TX and receiving portion RX.

Transmitting portion TX includes a first voltage driver 110, a first current driver 120 coupled with first voltage driver 110, a second voltage driver 130, a second current driver 140 coupled with second voltage driver 130, a control unit 150 coupled to first current driver 120 and second current driver 140 through bus 162, and a reference voltage unit 166 coupled to the control unit 150. Transmitting portion TX further includes a first power node 172, a second power node 174, a third power node 176, and a fourth power node 178. Channel portion CH includes a first channel 182 coupled to first voltage driver 110 and a second channel 186 coupled to second voltage driver 130. Receiving portion RX includes a first termination impedance 192 coupled to the first channel 182, a second termination impedance 196 coupled to the second channel 186, and a fifth power node 198 coupled to the first and second termination impedances 192 and 196.

First voltage driver 110 includes a P-type transistor 112, an N-type transistor 114, resistors 116, 117, and 118, an input node IN1, and an output node OUT1. P-type transistor 112 has a source coupled to first power node 172 through resistor 116. P-type transistor 112 has a drain coupled to a drain of N-type transistor 114. N-type transistor 114 has a source coupled to second power node 174 through resistor 117. Drains of transistors 112 and 114 are coupled to output node OUT1 through resistor 118. Gates of transistors 112 and 114 are coupled to input node IN1. In some embodiments, resistors 116, 117, and 118 are physical resistive devices. In some embodiments, one or more of resistors 116, 117, and 118 are not physical resistive devices. Rather, the one or more of resistors 116, 117, and 118 are used in the circuit diagram to represent resistance of transistors 112 and 114 and/or conductive lines of first voltage driver 110 observable from output node OUT1.

First power node 172 is configured to carry a first supply voltage having a first voltage level VDD. Second power node 174 is configured to carry a second supply voltage having a second voltage level VSS or ground GND less than the first voltage level VDD.

In some embodiments, first voltage driver 110 is configured to electrically couple output node OUT1 to first power node 172 when a first input signal at input node IN1 is at a first logic state, such as a logic low state. In some embodiments, first voltage driver 110 is configured to electrically couple output node OUT1 to second power node 174 when the first input signal at input node IN1 is at a second logic state, such as a logic high state.

In some embodiments, channel 182 has a predetermined intrinsic impedance Z₀. In some embodiments, first voltage driver 110 is configured to have an output impedance matching the predetermined intrinsic impedance Z₀. In some embodiments, output resistance of first voltage driver 110 when the first input signal at input node IN1 is at the logic low state that is equal to a resistance of the combination of resistor 116, resistor 118, and a turned-on resistance of transistor 112 in series. In some embodiments, output resistance of first voltage driver 110 when the first input signal at input node IN1 is at the logic high state that is equal to a resistance of the combination of resistor 117, resistor 118, and a turned-on resistance of transistor 114 in series.

First current driver 120 includes a P-type transistor 122, an N-type transistor 124, an inverter 125, a first set of current sources 126, and a second set of current sources 128. P-type transistor 122 has a source coupled to third power node 176 through first set of current sources 126. P-type transistor 122 has a drain coupled to a drain of N-type transistor 124. N-type transistor 124 has a source coupled to fourth power node 178 through second set of current sources 128. Drains of transistors 122 and 124 are coupled to output node OUT1. In some embodiments, third power node 176 is configured to carry a third supply voltage having a third voltage level VDDA, or to be coupled to the first power node 172. Fourth power node 178 is configured to carry a fourth supply voltage having a fourth voltage level VSSA or ground GND less than the third voltage level VDDA, or to be coupled to the second power node 174.

Inverter 125 has an input coupled to input node IN1 and an output. P-type transistor 122 has a gate coupled to input node IN1, and N-type transistor 124 has a gate coupled to output of inverter 125.

FIG. 2A is a circuit diagram of a voltage driver 110 and a current driver 120 having an input signal at a logic high state in accordance with one or more embodiments. Components similar to those depicted in FIG. 1 are denoted with the same reference numbers.

When the first signal is at the logic high state, transistors 122 and 124 are turned off (depicted as being covered by shadowed regions), and first current driver 120 is electrically decoupled from output node OUT1. Also, transistor 112 is turned off (depicted as being covered by a shadowed region), transistor 114 is turned on, and thus output node OUT1 is electrically coupled to power node 174. If power nodes 174, 178, and 198 have a voltage level set to be 0V, a voltage level at the output node OUT1 is 0V.

FIG. 2B is a circuit diagram of a voltage driver 110 and a current driver 120 having an input signal at a logic low state in accordance with one or more embodiments. Components similar to those depicted in FIG. 1 are denoted with the same reference numbers.

When the first signal is at the logic low state, transistors 122 and 124 are turned on, and first set of current sources 126 and second set of current sources 128 are electrically coupled to output node OUT1. Also, transistor 112 is turned on and transistor 114 is turned off (depicted as being covered by a shadowed region), and thus output node OUT1 is electrically coupled to power node 172.

Under this scenario, first current driver 120 is configured to inject or extract a first adjustment current into or out of output node OUT1. For example, when transistors 122 and 124 are turned on, first set of current sources 126 is coupled to output node OUT1 and configured to inject a first current I₁ into output node OUT1 responsive to a first set of control signals from the control unit 150 through bus 162 a. In some embodiments, when transistors 122 and 124 are turned on, second set of current sources 128 is coupled to output node OUT1 and configured to extract a second current I₂ from output node OUT1 responsive to a second set of control signals from the control unit 150 through bus 162 b. Therefore, an amount of adjustment current I injected into output node OUT1 equals (I₁−I₂), where a positive value of I denotes the injection of adjustment current and negative values of I denotes extraction of adjustment current. In some embodiments, one of current I₁ or I₂ is set to 0 for minimizing power consumption of the first current driver 110.

First termination impedance 192 of receiving portion RX is used to model the input impedance of a receiver. In some embodiments, first termination impedance 192 also has an input impedance matching the predetermined intrinsic impedance Z₀. In some embodiments, if R₀ represents a real part of the predetermined intrinsic impedance Z₀, I represents the amount of current injected into output node OUT1 by first current driver 120, and the voltage level VSS or ground GND is set to be 0V, the voltage level at output node OUT1 has a voltage level of (VDD/2+IR₀/2).

Therefore, as depicted in FIG. 2A and FIG. 2B, when voltage driver 110, current driver 120, channel 182, and termination impedance 192 are configured as advanced above, the voltage level at output node OUT1 has a signal swing equal to (VDD/2+IR₀/2). The signal swing at output node OUT1 is adjustable through controlling adjustment current I.

As depicted in FIG. 1, second voltage driver 130 and second current driver 140 are configured in a manner that would have substantially similar functionality as that of first voltage driver 110 and first current driver 120 illustrated above.

Second voltage driver 130 includes a P-type transistor 132, an N-type transistor 134, resistors 136, 137, and 138, an input node IN2, and an output node OUT2. P-type transistor 132 has a source coupled to first power node 172 through resistor 136. P-type transistor 132 has a drain coupled to a drain of N-type transistor 134. N-type transistor 134 has a source coupled to second power node 174 through resistor 137. Drains of transistors 132 and 134 coupled to output node OUT2 through resistor 138. Gates of transistors 132 and 134 are coupled to input node IN2. In some embodiments, resistors 136, 137, and 138 are physical resistive devices. In some embodiments, one or more of resistors 136, 137, and 138 are used in the circuit diagram to represent resistance of transistors 132 and 134 and/or conductive lines of first voltage driver and thus are not physical resistive devices.

In some embodiments, second voltage driver 130 is configured to electrically couple output node OUT2 to first power node 172 when a second input signal at input node IN2 is at the logic low state. In some embodiments, second voltage driver 130 is configured to electrically couple output node OUT2 to second power node 174 when the second input signal at input node IN2 is at the logic high state.

In some embodiments, channel 186 also has the predetermined intrinsic impedance Z₀. In some embodiments, second voltage driver 130 is configured to have an output resistance matching the predetermined intrinsic impedance Z₀ in a manner similar to that of first voltage driver 110.

Second current driver 140 includes a P-type transistor 142, an N-type transistor 144, an inverter 145, a third set of current sources 146, and a fourth set of current sources 148. P-type transistor 142 has a source coupled to third power node 176 through third set of current sources 146. P-type transistor 142 has a drain coupled to a drain of N-type transistor 144. N-type transistor 144 has a source coupled to fourth power node 178 through fourth set of current sources 148. Drains of transistors 142 and 144 coupled to output node OUT2. Inverter 145 has an input coupled to input node IN2 and an output. P-type transistor 142 has a gate coupled to input node IN2, and N-type transistor 144 has a gate coupled to output of inverter 145.

Similar to first current driver 120, second current driver 140 is configured to inject or extract a second adjustment current into or out of output node OUT2 when a second input signal at input node IN2 is at the logic low state. Under this circumstance, in some embodiments, third set of current sources 146 is configured to inject a third current I₃ into output node OUT2 responsive to a third set of control signals from the control unit 150 through bus 162 c. In some embodiments, fourth set of current sources 148 is configured to extract a fourth current I₄ from output node OUT4 responsive to a second set of control signals from the control unit 150 through bus 162 d.

In some embodiments, the second adjustment current is set to have the same amount of current flowing along consistent direction (i.e., injecting or extracting adjustment currents) as those of the first adjustment current. In some embodiments, the amount of adjustment current I injected into output node OUT2 equals (I₃−I₄), where a positive value of I denotes the injection of adjustment current and negative values of I denotes extraction of adjustment current. In some embodiments, one of current I₃ or I₄ is set to 0 for minimizing power consumption of the second current driver.

In some embodiments, first set of current sources 126 and third set of current sources 146 have similar configurations, and current I₁ and I₃ are set to be substantially the same. In some embodiments, second set of current sources 128 and fourth set of current sources 148 have similar configurations, and current I₂ and I₄ are set to be substantially the same.

If R₀ represents a real part of the predetermined intrinsic impedance Z₀, I represents the amount of current injected into output node OUT2 by second current driver 140, and the voltage level VSS or ground GND is set to be 0V, the voltage level at second output node OUT2 has a signal swing equal to (VDD/2+IR₀/2).

In some embodiments, first voltage driver 110 and second voltage driver 130 are configured to transmit a signal in a differential manner. Therefore, first input signal and second input signal are a pair of differential signals and have logically complementary states. Accordingly, in some embodiments, first voltage driver 110, first current driver 120, second voltage driver 130, and second current driver 140 is set to have a signal swing at output nodes OUT1 and OUT2 equal to (VDD+IR₀).

Control unit 150 has an analog-to-digital converter (ADC) 152 and a current adjusting unit 154. Control unit 150 is also coupled to power node 172 in order to receive the first supply voltage and reference voltage unit 166 in order to receive a reference voltage. ADC 152 is configured to generate a measurement result of the first voltage level of the first supply voltage based on the reference voltage. Current adjusting unit 154 is configured to output a set of control signals to the first current driver 120 to set the amount and the direction of the first adjustment current and to the second current driver 140 to set the amount and the direction of the second adjustment current according to the measurement result.

For example, when ADC 152 returns a measurement result of the first voltage level of the first supply voltage being VDD, and the signal transmission system 100 is set to have a predetermined differential mode output swing of Vx, current adjusting unit 154 is capable of setting current I₁, I₂, I₃, and I₄ to provide adjustment current I such that: Vx=(VDD+IR ₀), and thus IR ₀=(Vx−VDD).

In some embodiments, current adjusting unit 154 determines the control signals for controlling the current drivers 120 and 140 according to a predetermined look-up table using the measurement result VDD and the predetermined output signal swing Vx as indices.

Control unit 150 further has a storage unit 156 configured to store the set of control signals and/or the look-up table. Therefore, the measurement performed by ADC 152 and the determination of the set of control signals performed by current adjusting unit 154 need not be continuously and/or recursively executed. In some embodiments, control unit 150 is configured to perform measurement of the first supply voltage and determination of the set of control signals when signal transmission system 100 is powered on, is reset according to an external reset instruction, or is requested to re-execute the measurement and determination of the set of control signals according to an external calibration instruction.

In some embodiments, control unit 150 has a voltage divider 158 configured to receive and convert the first supply voltage from power node 172 into a divided voltage signal and output the divided voltage signal to ADC 152. In some embodiments, reference voltage unit 166 is configured to output the reference voltage based on a bandgap reference voltage. In some embodiments, ADC 152 is capable of converting a voltage signal having a voltage level between 0V to the reference voltage to a digital code.

In some embodiments, if the voltage level at power node 172 is close or above the reference voltage, voltage divider 158 then is usable to output the divided voltage signal that would fall within the applicable operation range of ADC 152. In some embodiments, ADC 152 is capable of converting only a voltage signal that has a voltage level falling within a smaller voltage range within the range of 0V to the reference voltage to the digital code, because the voltage level of power node 172 outside the smaller voltage range would cause current drivers 120 and 140 to be inoperable. In some embodiments, ADC 152 is capable of output digital codes that have a resolution of 5 to 30 mV each quantization step. In some embodiments, ADC 152 is configured to output a binary value having 3 bits to 7 bits.

FIG. 3A is a circuit diagram of a set of current sources 300A usable as a set of current sources 126 or 146 in a current driver 120 or 140 in accordance with one or more embodiments. Current sources 300A includes a reference current source 302, a diode-connected P-type transistor 304, a plurality of P-type transistors 312 a, 312 b, 312 c, and 312 d, and a plurality of switching devices 314 a, 314 b, 314 c, and 314 d.

Reference current source 302 is configured to provide a constant reference current. Sources of transistors 312 a, 312 b, 312 c, and 312 d and transistor 304 are coupled to power node 176. Gates of transistors 312 a, 312 b, 312 c, and 312 d and transistor 304 are coupled together. Transistors 312 a, 312 b, 312 c, and 312 d and transistor 304 are configured as a current mirror array such that transistors 312 a, 312 b, 312 c, and 312 d are capable of outputting corresponding current having predetermined current levels based on mirroring of the reference current from reference current source 302. In some embodiments, the corresponding current levels are determined according to channel width/length ratios of transistors 312 a, 312 b, 312 c, and 312 d and transistor 304. In some embodiments, transistors 312 a, 312 b, 312 c, and 312 d are configured to output the same amount of current. In some embodiments, transistors 312 a, 312 b, 312 c, and 312 d are configured to output different amounts of current. In some embodiments, there are more or fewer than four transistors 312 a, 312 b, 312 c, and 312 d for mirroring currents based on transistor 304.

The plurality of switching devices 314 a, 314 b, 314 c, and 314 d connects corresponding transistors 312 a, 312 b, 312 c, and 312 d to node 320 and is set to be turned on or off based on control signals from control unit 150 through bus 162 a or 162 c. In some embodiments, switching devices 314 a, 314 b, 314 c, and 314 d are also formed by P-type transistors. In some embodiments, each transistor 312 a, 312 b, 312 c, and 312 d is configured to output the same amount of current, and the set of control signals for controlling gates of transistors 312 a, 312 b, 312 c, and 312 d represents a value in a thermometer code format. In some embodiments, gates of transistor 312 a, 312 b, 312 c, and 312 d are coupled as groups of transistors each represent a corresponding power of two time(s) a unit current, and the set of control signals for controlling corresponding groups of transistors 312 a, 312 b, 312 c, and 312 d represents a value in a binary code format.

FIG. 3B is a circuit diagram of another set of current sources 300B usable as a set of current sources 128 or 148 in a current driver 120 or 140 in accordance with one or more embodiments. Current sources 300B includes a reference current source 332, a diode-connected N-type transistor 334, a plurality of N-type transistors 352 a, 352 b, 352 c, and 352 d, and a plurality of switching devices 354 a, 354 b, 354 c, and 354 d.

Reference current source 332 is configured to provide a constant reference current. Sources of transistors 352 a, 352 b, 352 c, and 352 d and transistor 334 are coupled to power node 178. Gates of transistors 352 a, 352 b, 352 c, and 352 d and transistor 334 are coupled together. Transistors 352 a, 352 b, 352 c, and 352 d and transistor 334 are configured as a current mirror array such that transistors 352 a, 352 b, 352 c, and 352 d are capable of outputting corresponding current having predetermined current levels based on mirroring of the reference current from reference current source 332. In some embodiments, the corresponding current levels are determined according to channel width/length ratios of transistors 352 a, 352 b, 352 c, and 352 d and transistor 334. In some embodiments, transistors 352 a, 352 b, 352 c, and 352 d are configured to output the same amount of current. In some embodiments, transistors 352 a, 352 b, 352 c, and 352 d are configured to output different amounts of current. In some embodiments, there are more or fewer than four transistors 352 a, 352 b, 352 c, and 352 d for mirroring currents based on transistor 334.

The plurality of switching devices 354 a, 354 b, 354 c, and 354 d connects corresponding transistors 352 a, 352 b, 352 c, and 352 d to node 360 and is set to be turned on or off based on control signals from control unit 150 through bus 162 b or 162 d. In some embodiments, switching devices 354 a, 354 b, 354 c, and 354 d are also formed by N-type transistors. In some embodiments, each transistor 352 a, 352 b, 352 c, and 352 d is configured to output the same amount of current, and the set of control signals for controlling gates of transistors 352 a, 352 b, 352 c, and 352 d represents a value in a thermometer code format. In some embodiments, gates of transistor 352 a, 352 b, 352 c, and 352 d are coupled as groups of transistors each represent a corresponding power of two time(s) a unit current, and the set of control signals for controlling corresponding groups of transistors 352 a, 352 b, 352 c, and 352 d represents a value in a binary code format.

FIG. 4 is a flow chart of a method 400 of operating a transmitter in accordance with one or more embodiments. It is understood that additional operations may be performed before, during, and/or after the method 400 depicted in FIG. 4, and that some other processes may only be briefly described herein.

As depicted in FIG. 1 and FIG. 4, in operation 410, control unit 150 generates a measurement result of a first voltage level of a first supply voltage at power node 172, such as VDD. In some embodiments, ADC 152 receives the first supply voltage at power node 172 and output a digital code representing the measurement result VDD based on a reference voltage from reference voltage unit 166. In some embodiments, voltage divider 158 receives the first supply voltage at power node 172 and converts the first supply voltage into a divided voltage signal. ADC 152 then receives the divided voltage generate the measurement result based on the divided voltage signal.

As depicted in FIG. 1 and FIG. 4, in operation 420, control unit 150 sets a current driver, such as current driver 120 and/or current driver 140 to provide an adjustment current based on the measurement result VDD. In some embodiments, operation 420 includes operation 422, where a set of current sources 126 of current driver 120 and/or a set of current sources 146 of current driver 140 is configured to inject a first current into the corresponding output node OUT1 or OUT2 responsive to a first set of control signals. In some embodiments, operation 420 further includes operation 424, where a set of current sources 128 of current driver 120 and/or a set of current sources 148 of current driver 140 is configured to extract a second current from the corresponding output node OUT1 or OUT2 responsive to a second set of control signals.

In some embodiments, the control signals for controlling the current drivers 120 and 140 are stored in a storage unit 156. In some embodiments, current adjusting unit 154 determines the control signals for controlling the current drivers 120 and 140 according to a predetermined look-up table using the measurement result VDD and a predetermined output signal swing as indices as illustrated above.

As depicted in FIG. 1 and FIG. 4, in operation 430, an input signal is received by voltage driver 110 and/or 130. In some embodiments, voltage driver 110 or 130 is configured to electrically couple a corresponding output node OUT1 or OUT2 to power node 172 when the input signal is at a logic low state, and to electrically couple the corresponding output node OUT1 or OUT2 to power node 174 when the input signal is at a logic high state.

As depicted in FIG. 1 and FIG. 4, in operation 440, current driver 120 or 140 determines if the corresponding input signal is at the logic high state or the logic low state. As depicted in FIG. 2B and FIG. 4, in operation 450, when the corresponding input signal is at the logic low state, the adjustment current from current driver 120 or 140 is applied to the output node OUT1 or OUT2. As depicted in FIG. 2A and FIG. 4, in operation 460, when the corresponding input signal is at the logic high state, current driver 120 or 140 electrically decoupling the current driver from the output node OUT1 or OUT2.

Therefore, according to one or more embodiments of the present application, output signal swing is set to a predetermined signal swing as set forth by a predetermined industrial standard. Also, variation of the power supply voltage of the corresponding voltage driver is capable of being compensated, and the output signal swing is calibrated, using the ADC 152 and the corresponding current driver.

In accordance with one embodiment, a circuit includes a first power node, a second power node, a first voltage driver, a first current driver, and a control unit. The first power node is configured to carry a first supply voltage having a first voltage level. The second power node is configured to carry a second supply voltage having a second voltage level less than the first voltage level. The first voltage driver has a first input node and a first output node. The first voltage driver is configured to electrically couple the first output node to the first power node when a first input signal at the first input node is at a first logic state, and electrically couple the first output node to the second power node when the first input signal at the first input node is at a second logic state. The first current driver is configured to inject or extract a first adjustment current into or out of the first output node. The control unit is configured to generate a measurement result of the first voltage level, and to set an amount and a direction of the first adjustment current according to the measurement result.

In accordance with another embodiment, a circuit includes a current driver, an analog-to-digital converter (ADC), and a current adjusting unit. The current driver is configured to inject or extract an adjustment current into or out of a node. The ADC is configured to generate a measurement result of a voltage level of a supply voltage based on a reference voltage. The current adjusting unit is configured to output a set of control signals to the current driver to set an amount and a direction of the adjustment current according to the measurement result.

In accordance with another embodiment, a method of operating a circuit including generating a measurement result of a first voltage level of a first supply voltage. A current driver is set to provide an adjustment current based on the measurement result. An input signal is received by a voltage driver. The voltage driver is configured to electrically couple an output node of the voltage driver to a first power node configured to carry the first supply voltage when the input signal is at a first logic state, and electrically couple the output node of the voltage driver to a second power node configured to carry a second supply voltage having a second voltage level less than the first voltage level when the input signal is at a second logic state. The adjustment current is applied to the output node of the voltage driver when input signal is at the first logic state.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A circuit, comprising: a first power node configured to carry a first supply voltage having a first voltage level; a second power node configured to carry a second supply voltage having a second voltage level less than the first voltage level; a first voltage driver comprising a first input node and a first output node, the first voltage driver being configured to electrically couple the first output node to the first power node when a first input signal at the first input node is at a first logic state, and electrically couple the first output node to the second power node when the first input signal at the first input node is at a second logic state; a first current driver configured to be electrically coupled to the first output node to inject or extract a first adjustment current into or out of the first output node and to be electrically decoupled from the first output node when the first input signal at the first input node is at the second logic state; and a control unit configured to generate a measurement result of the first voltage level, and to set an amount and a direction of the first adjustment current according to the measurement result.
 2. The circuit of claim 1, wherein the first voltage driver is configured to have an output impedance matching a predetermined intrinsic impedance of a channel.
 3. The circuit of claim 1, wherein the control unit comprises: an analog-to-digital converter (ADC) configured to generate the measurement result based on a reference voltage; and a current adjusting unit configured to output a set of control signals to the first current driver to set the amount and the direction of the first adjustment current according to the measurement result.
 4. The circuit of claim 3, wherein the control unit further comprises: a voltage divider configured to convert the first supply voltage into a divided voltage signal and output the divided voltage signal to the ADC.
 5. The circuit of claim 3, further comprising a reference voltage unit configured to output the reference voltage based on a bandgap reference voltage.
 6. The circuit of claim 1, wherein the first current driver comprises: a first set of current sources coupled to the first output node, the first set of current sources being configured to inject a first current into the first output node responsive to a first set of control signals from the control unit.
 7. The circuit of claim 6, wherein the first current driver further comprises: a second set of current sources coupled to the first output node, the second set of current sources being configured to extract a second current out of the first output node responsive to a second set of control signals from the control unit, and the first adjustment current being a difference of the first current and the second current.
 8. The circuit of claim 6, wherein the control unit comprises a storage unit configured to store the first set of control signals.
 9. The circuit of claim 1, further comprising: a second voltage driver comprising a second input node and a second output node, the second voltage driver being configured to electrically couple the second output node to the first power node when a second input signal at the second input node is at the first logic state, and electrically couple the second output node to the second power node when the second input signal at the second input node is at the second logic state; and a second current driver configured to inject or extract a second adjustment current into or out of the second output node, wherein the control unit is further configured to set an amount and a direction of the second adjustment current according to the measurement result.
 10. A circuit, comprising: a current driver configured to be electrically coupled to a node to inject or extract an adjustment current into or out of the node, the current driver comprising switches configured to electrically decouple the current driver from the node; an analog-to-digital converter (ADC) configured to generate a measurement result of a voltage level of a supply voltage based on a reference voltage; and a current adjusting unit configured to output a set of control signals to the current driver to set an amount and a direction of the adjustment current according to the measurement result.
 11. The circuit of claim 10, wherein the control unit further comprises: a voltage divider configured to convert the supply voltage into a divided voltage signal and output the divided voltage signal to the ADC.
 12. The circuit of claim 10, further comprising a reference voltage unit configured to output the reference voltage based on a bandgap reference voltage.
 13. The circuit of claim 10, wherein the current driver comprises: a first set of current sources coupled to the node, the first set of current sources being configured to inject a first current into the node responsive to the set of control signals.
 14. The circuit of claim 13, wherein the current driver further comprises: a second set of current sources coupled to the node, the second set of current sources being configured to extract a second current out of the node responsive to the set of control signals, and the adjustment current being a difference of the first current and the second current.
 15. The circuit of claim 10, wherein the control unit comprises a storage unit configured to store the set of control signals.
 16. A method of operating a circuit, comprising: generating a measurement result of a first voltage level of a first supply voltage; setting a current driver to provide an adjustment current based on the measurement result; receiving an input signal by a voltage driver, the voltage driver being configured to electrically couple an output node of the voltage driver to a first power node configured to carry the first supply voltage when the input signal is at a first logic state, and electrically couple the output node of the voltage driver to a second power node configured to carry a second supply voltage having a second voltage level less than the first voltage level when the input signal is at a second logic state; electrically coupling the current driver to the output node and applying the adjustment current to the output node of the voltage driver when the input signal is at the first logic state; and electrically decoupling the current driver from the output node when the input signal is at the second logic state.
 17. The method of claim 16, wherein the current driver comprises a first set of current sources coupled to the output node, and the setting the current driver comprises configuring the first set of current sources to inject a first current into the output node responsive to a first set of control signals.
 18. The method of claim 17, wherein the current driver further comprises a second set of current sources coupled to the output node, and the setting the current driver further comprises configuring the second set of current sources to extract a second current from the output node responsive to a second set of control signals.
 19. The method of claim 16, wherein the generating the measurement result comprises: converting the first supply voltage into a divided voltage signal; and generating the measurement result based on the divided voltage signal.
 20. The method of claim 16, wherein setting the current driver to provide the adjustment current based on the measurement result comprises determining a set of control signals for controlling the current driver according to a predetermined look-up table. 